One known application for radio frequency identification (RFID) technology is to track a mobile object, such as a shipping container. A device known as a “tag” is provided on the object to be tracked. The tag typically includes circuitry coupled to an antenna, and a battery to power the circuitry. The tag can transmit radio signals, and some tags can also receive radio signals. Existing tags have been generally adequate for their intended purposes, but have not been satisfactory in all respects.